This invention relates to a control system for controlling and stabilizing the operation of a damperless synchronous motor during transients or disturbances, as well as during steady state operation, to stabilize the motor's torque angle and maintain it within the stability limit.
In a synchronous motor, whether of the wound rotor (electromagnetic) type or permanent magnet rotor type, or brushless type, a rotating magnetic stator field produced by a set of stator windings causes the rotor to rotate in step or in synchronism with the stator field, the rotor speed or frequency thereby equalling the stator field frequency. The magnetic poles of the rotor are attracted by the revolving stator field and follow it in absolute synchronism producing torque by virtue of the magnetic interactions. There is no slip. The torque angle, namely the angle between the applied stator voltage and the no load back emf induced in the stator, may change somewhat (increasing with increasing mechanical load on the motor and decreasing with decreasing load) but the frequencies of the stator and rotor voltages will remain the same. Typically, the torque angle may be close to 0.degree. at no load, and anywhere from 20.degree. to 60.degree. at full load. If too much mechanical load is added, the motor's torque angle increases to the extent that synchronous operation is lost. The angle at which this occurs is called the "stability limit" and is determined by the motor's parameters as well as the load and the voltage applied to the motor. For example, in some synchronous machines the stability limit may be around 90.degree.. It is most desirable that the torque angle be held within the stability limit because if the limit is exceeded and the motor pulls out of synchronism (referred to as a "fault condition") the resulting transient torques and currents may be destructive.
Line voltage or load torque disturbances or transients may cause the rotor of a synchronous motor to hunt or oscillate as the rotor rotates. The oscillations may become so great that the stability limit is exceeded, this being particularly true when sudden load torque changes occur. For this reason, devices and systems of various types have been developed in the past to minimize the effects of these transients and disturbances in order to stabilize the operation of synchronous machines. One approach has been to employ a damper which is a shorted winding on the rotor. Such a damper winding tends to reduce the magnitude of any hunting or oscillation of the rotor.
The advent of the high powered static frequency changer has made it possible to design synchronous motor drive systems that do not require damper windings at all because the necessary damping function may now be provided electronically. The synchronous motor systems developed up to this time generally use forced commutated current source or voltage source inverters (some varieties of which are called brushless DC motor drives) or motor commutated, current source inverters. New in the art, as described by my companion patent application, Ser. No. 452,460, filed concurrently herewith, and entitled "Voltage-Controlled, Inverter-Motor System", are motor-commutated voltage source inverters which permit the design of very simple and inexpensive motor drives. The synchronous motors used with current source inverters still require damper windings to aid in the commutation process whereas the synchronous motors used with voltage source inverters do not require dampers for that purpose.
The elimination of the damper winding in the motor used in the voltage source drives provides several advantages. First, the motor becomes smaller, lighter, and less expensive. Secondly, the impedance to hamonic voltages generated by the non-sinusoidal inverter voltage is raised considerably, resulting in less harmonic current flow and an attendant reduction of harmonic loss. Therefore, motor efficiency is increased and cooling of the motor (particularly the rotor) becomes easier.
Prior to this invention, stabilization of a synchronous motor, operated from variable-frequency, voltage-controlled forced commutated inverters, has been obtained without a damper by using a shaft position sensor which is mounted on the motor shaft and effectively determines the position of the rotor flux or magnetic field. A signal from the shaft position sensor controls the inverter frequency to keep the stator mmf (magnetomotice force) from getting too far ahead of the rotor mmf. By "slaving" the inverter frequency to the position of the rotor, as determined by the sensor, the stator and rotor magnetic fields are maintained sufficiently close together to hold the torque angle relatively stable within the stability limit despite the presence of transients, disturbances, or sudden load torque changes. While such shaft position sensors are effective in providing stable drives, they do increase the cost and complexity of the synchronous motor and, moreover, if the system is hermetically sealed, wires from the sensor must be brought out through the sealed system.
As previously mentioned, motor-commutated voltage source inverters were unheard of prior to the development described in my co-pending application. Stabilization of the motors used in these system, although possible by the use of prior art position sensors may also easily be accomplished by the techniques presently being disclosed. The control system of the present invention constitutes a significant improvement over these prior systems in that stabilization of a synchronous motor, operating from a voltage source inverter, is achieved without requiring a damper or a motor shaft position sensor. The present invention maintains a stable torque angle well within the motor's stability limit by means of a very efficient system which is considerably simpler and less expensive in construction than the previous systems.